1. Field of the Invention
The invention relates generally to electrolytic processes and apparatus for refining metals and, in particular, to an improved stainless-steel clad electrode.
2. Description of the Prior Art
The principle of electrolysis has been utilized for decades to extract metals and other cations from an electrolytic solution. The extraction process is carried out by passing an electric current through an electrolyte solution of the metal of interest, such as copper, gold, silver, or lead. The metal is extracted by electrical deposition as a result of current flow between a large number of anode and cathode plates immersed in cells of a dedicated extraction tank house. In electrowinning processes, a solution of metal-rich electrolyte and various other chemicals, as required to maintain an optimal rate of deposition, is circulated through the extraction cells. The cathode is generally constructed of a metal alloy, such as titanium or copper alloys, and various grades of stainless steel which are resistant to corrosive acid solutions. In the most efficient processes, each cathode consists of a thin sheet of metal of uniform thickness, e.g., 2-4 mm, disposed vertically between parallel sheets of anodic material, so that a uniform current density is present throughout the surface of the cathode. On passing of an electric current through the anodes, electrolyte and cathodes, a pure layer of metal is electrodeposited on the cathode surface which becomes plated during the process.
Similarly, in a metal purification process in a refinery using electrodeposition, an anode of impure metal is placed in an electrolytic solution of the same metal and subjected to an electric current passing through the anode, electrolyte and cathode of each cell. The anode goes into solution and the impurities drop to the bottom of the tank. The dissolved metal then follows the current flow and is deposited in pure form on the cathode which typically consists of a mother plate of stainless steel. When a certain amount of pure metal has been plated onto the mother plate, the cathode is pulled out of the tank and stripped of the pure metal.
In both processes, the pure metal deposit is grown to a specific thickness on the cathode during a predetermined length of time and then the cathode is removed from the cell. It is important that the layer of metal deposited be recovered in uniform shapes and thicknesses and that its grade be of the highest quality so that it will adhere to the cathode blank during deposition and be easily removed by automated stripping equipment afterwards. The overall economy of the production process depends in part on the ability to mechanically strip the cathodes of the metal deposits at high throughputs and speeds without utilizing manual or physical intervention. To that end, the cathode blanks must have a surface finish that is resistant to the corrosive solution of the tank house and must be strong enough to withstand continuous handling by automated machines without pitting or marking. Any degradation of the finish of the blank causes the electrodeposited metal to bond with the cathode resulting in difficulty of removal and/or contamination of the deposited metal.
Also immensely important in the production and refining of metals by electrolytic extraction is the relationship of electrical power consumption with rates of metal production. A cell efficiency of ninety-five percent or better is the typical goal for the best operations. In order to achieve this level of efficiency, the voltage profile across the cathodic deposition surface must be held constant and variations avoided. Shorts due to areas of high current density caused by nodulization or by curved cathode surfaces which touch the anode must be prevented. Therefore, the details of construction of cathode blanks are very important to minimize operational problems and ensure high yields.
U.S. Pat. No. 4,186,074 to Perry describes a cathode for the electrolytic refining of copper that consists of a stainless steel hanger bar which is point-welded to a stainless steel mother plate hanging from the bar in a vertical position, as illustrated in FIG. 1. The hanger bar has a flat bottom face for maximum surface contact and corresponding maximum electrical conductance with support bus-bars through which the system is energized. In order to reduce the electrical resistance resulting from the welds between the hanger bar and the mother plate, the hanger bar and the upper edge of the mother plate are uniformly clad with copper, thereby creating a low-resistance boundary between the bar and the sheet. Additionally, in order to prevent deposit build-up along the lateral edges of the mother plate which would impede the automated separation of the product at the end of each cycle, these lateral edges are masked with an insulating strip riveted to the electrode.
U.S. Pat. No. 5,492,609 to Assenmacher disclosed an improved cathode that includes a copper hanger-bar with a rounded underside that ensures the automatic vertical position of the mother plate with respect to horizontal supporting bus-bars irrespective of warpage or construction defects. The mechanical connection between the hanger bar and the mother plate is achieved by inserting the latter""s upper edge into a receiving groove in the underside of the hanger bar and by welding the entire length of connection, thereby establishing a large boundary surface for good electrical conductance.
Another development in the art is a hanger bar that consists of hollow stainless-steel tubing spot-welded to the mother plate in conventional manner. The hanger bar and the spot welds are then covered with plated copper to improve electrical conductance. The problem with this hanger-bar configuration is the fact that the copper plating tends to come off the spot welds with use, thereby creating current paths with different conductivity and nonuniform current densities.
Another type of electrode, developed primarily for use in chloride-rich environments, consists of a copper hanger-bar clad in titanium and welded to a titanium mother plate. A copper tube is first covered with a layer of titanium to produce the hanger bar. The mother plate is then either fingered around or welded to the hanger bar, or both, thereby requiring a more durable and efficient titanium-to-titanium connection between the two components. Because of the materials involved, this type of electrode is relatively expensive and its use is reserved to specialized applications.
Bartsch et al. (U.S. Pat. No. 4,647,358) have disclosed a cathode with a hanger bar consisting of a hollow copper tube clad in a tubular sheath of high grade steel. The steel sheath is preferably a tube with slightly larger diameter than the copper tube and is slidably fitted over it. The electrode""s mother plate, which is also made of steel, is then welded to the steel sheath cladding the hanger bar. In order to improve the contact between the copper and steel tube sections of the hanger bar, they may be deformed to an oval shape by the application of pressure. Additional contact pressure points may be provided, such as by center-punch blows, to improve current flow between the copper and steel tubes of the hanger bar. A tight seal between the copper and steel tubes also prevents diffusion of liquid and gas and ensures a satisfactory flow of current through the cross-sections of the welds.
While these inventions have provided substantial improvements over the prior art, the maintenance of a uniform current density remains a problem due to wear, pits or other faults developed in the electrode, especially at the points of connection between the hanger bar and the mother plate. The center-punch blows of the Bartsch et al. cathode tend to create stresses in the steel wrap that may cause corrosion attacks. Although its steel-clad copper hanger bar is very desirable to improve electrical conductivity with the mother plate, its open-ended tubular configuration allows contamination and acidic corrosion inside the hollow tube. In addition, a tubular hanger bar is mechanically weaker than a solid hanger bar and more susceptible to excessive heating caused by the high current demands of today""s tank houses, which in turn may cause electrical shorts and reduced quality of conductivity to the mother plate. All of these forms of damage exacerbate the normal damage caused by exposure to the corrosive atmosphere of the electrolytic tank house, rapidly leading to a build-up of high-resistance corrosion spots that decrease the conductivity of the entire electrode. Such corrosion eventually causes enough structural damage to require replacement of the hanger bar and reconditioning of the cathode.
These problems are particularly relevant for liberator and scavenger cells, which are specialized cells used to decrease the copper concentration in the electrolyte solution. In copper refining, the copper solution is processed under steady-state conditions at a concentration of approximately 44 grams of copper per liter of solution. As electroplating proceeds, the copper concentration tends to rise above optimal levels and it becomes necessary to xe2x80x9cliberatexe2x80x9d the excess electrolyte from the process solution. The copper concentration rises in similar fashion in electrowinning processes, creating a need for xe2x80x9cscavengingxe2x80x9d the excess copper from the solution. These operations are carried out in dedicated cells. Both liberator and scavenger cells can operate at relatively high amperages and unusually hot conditions, producing gases that corrode the exposed portions of the mother plate and, especially, the exposed copper of the hanger bar.
In view of the above, there still exists a need for an improved electrolytic cathode which overcomes these problems, especially with regard to liberator and scavenger cells. The present invention provides a new electrode configuration and a method of construction of such electrode that fulfill this need.
The primary objective of this invention is an electrode and particularly a cathode that has optimal and long-lasting electrical characteristics for the electrolytic production and refining of copper.
Another objective of the invention is an electrode wherein the bearing structure that couples the hanger bar to the mother plate includes a weld between stainless steel components, thereby simplifying the process of assembly and ensuring strength and durability.
Another goal is a cathode with a solid copper hanger bar that is reliably protected from exposure to corrosive gases produced in refining and electrowinning processes.
A further objective of the invention is a cathode that is particularly suitable for use in the high-concentration and corrosive-atmosphere conditions found in liberator and scavenger cells.
Another objective is a cathode that performs reliably when used with all types of automated mechanical stripping machines, cathode handling equipment, and various types of edge strips.
Another goal of the invention is an electrically efficient cathode assembly which is capable of receiving a maximum amount of deposited metal while being easily stripped and reused during the life of its components.
Yet another objective is a design and method of manufacture for an electrode and particularly a cathode that accomplishes the above-mentioned objects in an economical and commercially viable manner.
Therefore, according to these and other objectives, the present invention consists of an electrolytic cathode comprising a solid copper hanger bar and a stainless-steel mother plate coupled by inserting the upper edge of the mother plate into a receiving groove in the underside of the hanger bar. In the preferred embodiment of the invention, the entire length of connection is welded, thereby establishing a large boundary surface for good electrical conductance. A cover of stainless steel is wrapped over the copper bar and the upper portion of the mother plate, leaving only the ends of the solid copper bar exposed for connection to conventional bus-bars. The lower edges of the steel cover are attached to the mother plate by a stainless-steel to stainless-steel weld that produces a strong and durable connection. The lateral edges of the cover are attached around the perimeter of the copper bar by conventional deoxygenated copper welds, or equivalent molybdemum/copper or other alloy welds, to completely seal the cover around the copper bar and over its points of connection to the mother plate, thereby preventing contamination from the electrolytic solution and process gases.
According to another aspect of the invention, the step of wrapping the steel cover over the solid copper hanger bar is carried out by molding, mechanically folding, stamping, or roll forming the cover around the copper bar, such as to produce intimate contact between the copper and steel surfaces. The cover is then welded to the mother plate and sealed around the copper bar. The heat produced by the welding process, which causes the steel cladding material to expand during welding, is also advantageously used to improve the tightness of the fit between the cover and the copper bar as a result of the cover""s shrinkage occurring during cooling.
Various other purposes and advantages of the invention will become clear from its description in the specification which follows and from the novel features particularly pointed out in the appended claims. Therefore, to the accomplishment of the objectives described above, this invention consists of the features hereinafter illustrated in the drawings, fully described in the detailed description of the preferred embodiments and particularly pointed out in the claims. However, such drawings and description disclose only some of the various ways in which the invention may be practiced.